The first neutrinos have been spotted colliding with heavy water molecules in a giant tank at the bottom of an Ontario nickel mine. Announced on Wednesday, the events mark the inauguration of the Sudbury Neutrino Observatory (SNO), a new facility that physicists hope will finally solve the solar neutrino problem, which has been haunting the field for decades.

Generated by the sun's nuclear processes, nearly a billion solar neutrinos--ghostly subatomic particles that can easily pass through Earth without hitting anything--shower down on each square centimeter of the planet's surface every second. Although neutrinos come in three "flavors," electron, muon, and tau, existing detectors can only see the electron variety, and they only see half as many as theorists had predicted. To resolve this discrepancy, physicists have proposed that half of the solar electron neutrinos switch flavors, or "oscillate," on their way from the sun's center to Earth. SNO was built to test this theory.

The key is its ability to see several varieties of solar neutrinos at once. SNO contains 1000 tons of ultrapure heavy water, water in which the hydrogen atoms have been replaced with deuterons, whose nuclei have a proton and a neutron. When an electron neutrino collides with a heavy water molecule, it can split apart the neutron and the proton and eject an electron. Other neutrino flavors split the nuclei but don't scatter electrons. By counting both neutrons and electrons, SNO should be able to measure both the total number of incoming neutrinos and the fraction of electron neutrinos, says physicist and SNO spokesman David Wark of Oxford University, England. If SNO finds that the shortfall of electron neutrinos is made up in other flavors, it will provide strong support for oscillations.

"It is an extremely important experiment," agrees physicist Paul Langacker of the University of Pennsylvania in Philadelphia. "They will very likely ascertain definitively whether neutrino oscillations are taking place." Unfortunately, physicists will have to be patient: Neutrinos collide with matter so rarely that SNO will detect only some 20 neutrinos every day. As a result, says Wark, "It will be at least a year" before SNO has an answer.